1. Field of the Invention
The present invention relates to an X-ray CT device that realizes cone-beam reconstruction, as well as a method of controlling the device. The present invention relates particularly to a medical X-ray CT device for radiating a living body with X-rays and thereby acquiring internal body information as an image.
2. Description of the Related Art
In the field of X-ray CT devices, the third-generation CT is defined as a system of collecting projection data from various angles as its X-ray tube for producing an X-ray beam and X-ray detector positioned across a subject from the X-ray tube rotate around the subject. The conventional technologies adopt an X-ray beam of a fan shape and a detector of a one-dimensional array type.
Scan systems can be divided into two types, conventional scan and helical scan. In the conventional scan, the X-ray tube moves around on a certain circular track. On the other hand, the helical scan is defined as the X-ray source and the detector continuously rotating around a subject while a bed carrying the subject moves along the body axis in synchronization with the rotation of the X-ray source and the detector. The helical scan has been given this name because the X-ray tube moves on a helical track when the coordinates of the tube moving along with the subject are considered. The distance between the changed positions of the X-ray tube along the body axis, in other words z-axially, after one rotation is defined as the helical pitch.
In addition, a CT incorporating a detector of a two-dimensional array type has been known as a third- or fourth-generation CT. This CT is provided with an X-ray tube that generates a conical X-ray beam spreading along the body axis, unlike fan-shaped X-rays, and an X-ray detector that is formed by attaching, for example, N rows of one-dimensional array detectors together z-axially so as to arrange detection elements in a matrix. Such a device is called a cone-beam CT scanner.
Among tomographic technologies employing conventional scan systems, a circular cone-beam reconstruction suggested by Feldkamp et al. is known for the reconstruction of an image obtained by the tomography (see Jpn. Pat. Appln. KOKAI Publication No. 2002-360562, for example). It has been shown, however, that the circular cone-beam reconstruction method produces cone-beam artifacts, which hampers production of complete data and degrades the image quality.
Solutions to this problem includes a line-and-circle (hereinafter referred to as line-circle) reconstruction technique, which employs line scan in addition to circular scan, as described in Jpn. Pat. Appln. KOKAI Publication No. 5-324801. Katsevich suggests line-circle reconstruction that adopts a filtered back projection system.
There is a drawback, however, in implementing such line-circle reconstruction on a system.
That is, additional scan is required to obtain line data. This is regarded as extra exposure to radiation, and the amount of exposure should be reduced as much as possible. Furthermore, the condition of collecting line data needs to match the condition of collecting circle data. If it does not match, artifacts may not be reduced. In addition, when the reconstruction is retried, preferable results cannot be attained unless the reconstruction is executed with suitable data combination.